The overall purpose of this project is to determine the cellular mechanisms that determine power output of me heart. A major focus of this proposal is determination of how variable expression of the two cardiac myosin heavy chain (MyHC) isoforms regulates contractile properties of cardiac myocytes. MyHC content will be manipulated over the entire range of 100% alpha-MyHC to 100% beta-MyHC by thyroid hormone-dependent expression of MyHC isoforms in rats. Definitive relationships between MyHC content and mechanical properties will be achieved by SDS-PAGE analysis of protein composition of individual myocytes following functional measurements. The specific aims of this study include determination of the effects of MyHC on Ca2+ sensitivity of stead:-state isometric force, loaded shortening velocity and power output. Another aim is to assess the chemomechanical steps of the myosin cross-bridge cycle that are most important in determining myocyte power output over a wide range of loads. These experiments will utilize reagents that increase the population of specific cross-bridge states, to examine how specific chemomechanical states alter power-load curves. Another aim is to determine if myocardial variations in force and power output in response to protein kinase A (PKA)-induced phosphorylation of myofibrillar proteins differ between alpha-MyHC and beta-MyHC myocytes and to determine whether phosphorylation of myosin binding protein-C is necessary for increased myofibrillar power output following beta-adrenergic stimulation. A final aim is determine the effects of PKCe-induced phosphorylation on myofibrillar power output. Overall, these experiments should provide important insights into the cellular and molecular factors that regulate power output in the heart and should improve our understanding the functional consequences of reduced expression of alpha-MyHC and altered phosphorylation states of myofibrillar proteins, which both occur during the progression of human heart failure.